Oxyalkylene polymers, particularly oxymethylene polymers having repeating oxymethylene units (i.e., --CH.sub.2 O-- are well known. Known polymerization processes for the preparation of oxyalkylene polymers are broadly classified into two processing techniques. One such process technique including polymerizing anhydrous formaldehyde as a principal monomer, while the other process technique including polymerizing, as a principal monomer, a cyclic acetal, such as trioxane (a cyclic trimer of formaldehyde).
With respect to the former processing technique, it has been proposed to (co)polymerize substantially anhydrous formaldehyde in the presence of an anionic or cationic catalyst. With respect to the latter processing technique, it has been proposed to carry out the (co)polymerization of a cyclic acetal (such as trioxane) as a principal monomer in the presence of a cationic catalyst. Examples of the catalysts which have been proposed include Lewis acids such as halides of boron, tin, titanium, phosphorus, arsenic and antimony, particularly, boron trifluoride, tin tetrachloride, titanium tetrachloride, phosphorus pentachloride, phosphorus pentafluoride, arsenic pentafluoride, antimony pentafluoride and complex compounds and salts thereof; protonic acids such as perchloric acid; protonic acid esters such as esters of perchloric acid with lower aliphatic alcohols, particularly t-butyl perchlorate; protonic acid anhydrides, particularly mixed acid anhydrides of perchloric acid will lower aliphatic carboxylic acids, such as acetyl perchlorate; trimethyloxonium hexafluorophosphate, triphenylmethyl hexafluoroarsenate, acetyl tetrafluoroborate, acetyl hexafluorophosphate and acetyl hexafluoroarsenate. Among them, boron fluoride and coordination compounds thereof with an organic compound such as an ether are most generally used as a catalyst for (co)polymerizing a cyclic oligomer of formaldehyde such as trioxane as a principal monomer and are frequently used in the (co)polymerization on an industrial scale. However, the (co)polymer prepared by the use of any of known catalysts as described above is limited in the degree of that is obtained. Thus, it is difficult according to conventional processes to obtain a (co)polymer having a degree of polymerization exceeding a certain limit by the use of the catalysts noted previously.
Furthermore, when a cyclic ether or formal having at least two adjacent carbon atoms is copolymerized with formaldehyde or a cyclic oligomer thereof by the process of the prior art for the purpose of introducing a stable unit into an acetal polymer, as copolymer generally contains a thermally unstable moiety at the terminal ends of its molecule. The copolymer must therefore be stabilized by eliminating the unstable moiety in order for it to be useable. The elimination of unstable moieties necessitates complicated post-treatment which consumes energy, and is thus uneconomical. On the other hand if the as-produced crude acetal copolymer copolymerization contains only a reduced amount of an unstable moiety, the resulting final product will exhibit improved stability thereby simplifying its post-treatment. Accordingly, the development of a polymerization process by which an acetal copolymer containing a reduced amount of unstable moiety can be prepared has been needed.
The cause of the above problems (i.e., a polymer or copolymer having a limited degree of polymerization and/or a significant amount of unstable ends is presumably that a) the known polymerization catalysts as described above not only accelerate the (co)polymerization, but also participate in the decomposition and depolymerization of the produced (co)polymer. Thus, the acetal (co)polymer prepared by the use of such a catalyst exhibits molecular weight, thermal stability, moldability and color which vary depending upon the specific type of the catalyst. In view of these problems, the present invention has been made and is based upon the discovery of a polymerization catalyst which overcomes the above noted problems.
The present invention broadly relates to a process for the preparation of an acetal polymer or copolymer which comprises polymerizing formaldehyde or a cyclic oligomer thereof alone, or copolymerizing it as a principal monomer with a comonomer copolymerizable therewith, and is especially characterized by the use of a heteropoly acid or an acid salt thereof as a polymerization catalyst.